An electroless plating is such a method that a plated film is formed on a plating surface of a workpiece by chemically reducing metal ions in a plating solution without supplying any electric current from the outside, and the electroless plating is widely used in a nickel-phosphorus plating and a nickel-boron plating for giving a corrosion resistance and a wear resistance, and a copper plating for a printed-wiring substrate.
As an electroless plating apparatus, there has been generally known an apparatus comprising a plating bath for holding an electroless plating solution which overflows during plating process, and a vertically movable holding portion disposed above the plating bath for holding a plating workpiece, such as a substrate, whereby the workpiece held by the holding portion is dipped into the plating solution in the plating bath. This kind of plating apparatus is provided with a plating solution regulating device separately for regulating the temperature, the components, etc. of the plating solution. The temperature and the components of the plating solution that has been overflowed from the plating bath is regulated in the plating regulating device, and then plating solution is supplied to the plating bath.
In recent years, as the processing speed and integration of a semiconductor chip becomes higher, there has been a growing tendency to replace aluminum or aluminum alloy with copper having a low electric resistivity and a high electromigration resistance as metallic materials for forming interconnection circuits on the semiconductor substrate. These kind of copper interconnects are generally formed by filling fine recesses formed in the surface of the substrate with copper. As a method for forming the copper interconnects, CVD, sputtering, and plating are known, but plating is generally used. In any case, after a copper film is deposited on the surface of the substrate, the surface of the substrate is polished to a flat finish to remove the plated copper on the surface of the substrate by a chemical mechanical polishing (CMP) process.
In the case of interconnects formed by such a process, the embedded interconnects have an exposed surface after the flattening processing. When an additional embedded interconnect structure is formed on such an exposed surface of interconnects of a semiconductor substrate, the following problems may be encountered. For example, during the formation of a new SiO2 interlevel dielectric, the exposed surface of the pre-formed interconnects is likely to be oxidized. Further, upon etching of the SiO2 layer for the formation of contact holes, the pre-formed interconnects exposed at the bottoms of the contact holes can be contaminated with an etchant, a peeled resist, etc. Moreover, in the case of copper interconnects, there is a fear of copper diffusion.
In view of this, in the case of copper interconnects, for example, it may be considered to selectively cover the surface of copper interconnects with a protective layer (plated film) of a Ni—B alloy or the like, having a good adhesion to copper and a low resistivity (ρ). The Ni—B alloy layer can be formed on the surface of e.g. copper selectively by using an electroless plating solution that contains nickel ions, a complexing agent for nickel ions and an alkylamine borane or a borohydride compound as a reducing agent for nickel ions and by immersing the surface of the substrate in the electroless plating solution.
An electroless plating is applied to main filling materials (Cu) for the copper interconnects, the formation of the seed layer on the barrier metal, or the reinforcement of the seed (Cu), to further the formation of the barrier metal itself, or the formation of cap material for the copper interconnect (in any case, Ni—P, Ni—B, Co—P, Ni—W—P, Ni—Co—P, Co—W—P, Co—W—B), or the like. In any electroless plating process, uniformity of the film thickness over an entire surface of the substrate is required.
In electroless plating, when a plating surface of a workpiece is brought into contact with an electroless plating solution, a plating metal instantly begins to deposit on the plating surface of the material, and the deposition rate of the plating metal varies depending on the temperature of the plating solution. Accordingly, in order to form a plated film having a uniform film thickness on the plating surface of a workpiece, the temperature of a plating solution is required to be uniform all over the surface of the material from the initial time of contact between the workpiece and the plating solution, and the uniform plating temperature must be kept throughout the plating treatment.
In conventional electroless plating apparatuses, a plating solution heated to a predetermined temperature is supplied to a plating bath or to the plating surface of a workpiece. However, the temperature of the plating solution is likely to change (lower) during transportation of the plating solution or in the course of plating. Even when the plating solution in the plating bath is heated by a heater or the like built in a substrate holder, it has been difficult to control the plating solution at a constant temperature throughout the plating treatment. Further, in order to secure the uniformity of plating over the entire plated surface, uniformity of the flow of plating solution in a plating bath is required in addition to the uniformity of plating temperature. In this connection, if a plating solution in a plating bath is always circulated or stirred, it is difficult to create a uniform flow of plating solution over the surface of a workpiece. Conversely, if a plating solution in a plating bath is not circulated or stirred, it is difficult to keep the temperature of the plating solution uniform throughout the solution.
The rate of electroless plating and the quality of plated film depend largely on the temperature of the electroless plating solution. In order to secure uniformity of the film thickness over the entire surface of a workpiece to be processed, it is desired to control the variation of the plating solution temperature within the range of ±1° C. over the entire surface of the workpiece to be processed. However, according to conventional electroless plating apparatuses, a temperature variation in the order of ±5° C. is generally produced during plating in the plating solution held in a plating bath, since the temperature of the plating solution is likely to change in the course of the plating process, and hence it is difficult to meet the ±1° C. variation requirement. Further, the plating apparatus occupies a considerably large space and is complicated, since the plating apparatus includes the plating bath and the plating solution regulating device. Electroplating apparatuses are the same as the above-described situation.